With the recent advancements in technology, there has been developed an electromagnetic band gap (hereinafter, referred to as EBG) structure. The EBG structure has been proposed, for example, as a means for preventing electromagnetic interference between circuits due to unwanted electromagnetic radiation from a high frequency circuit. In a broad sense, the EBG structure refers to a two- or three-dimensional periodic structure of dielectrics or conductors that suppresses or greatly attenuates two- or three-dimensional propagation of electromagnetic waves in a certain frequency band.
As configuration of the EBG structure, a high impedance surface (hereinafter, referred to as HIS) is disclosed in PTL 1 and others. Important characteristics include the suppression of surface waves (surface current) and the in-phase reflection of electromagnetic waves.
FIG. 1 illustrates a device that has a conventional HIS structure. FIG. 1(a) shows the sectional view of the HIS shown in FIG. 2a of PTL 1. FIG. 1(b) shows the top view of the HIS shown in FIG. 2b of PTL 1. FIG. 1(c) shows the top view of the HIS shown in FIG. 3a of PTL 2.
As shown in FIG. 1(a), the HIS 1 has the structure that conductor elements 4 of thumbtack shape each composed of a conductor piece 2 and a conductor post 7 are periodically arranged on a conductor plane 3. Each conductor element 4 is electrically connected to the conductor plane 3. The shapes of the conductor pieces 2 proposed include a regular hexagonal shape as shown in FIG. 1(b) and a square shape as shown in FIG. 1(c).
FIG. 2 shows an equivalent circuit diagram of the conventional HIS shown in FIG. 1 of PTL 1. The HIS of FIGS. 1(a) to 1(c) can be considered as a distributed constant circuit that includes a two-dimensional array of series capacitances C between adjoining thumbtack conductors and parallel inductances L each formed by two thumbtack conductors and the conductor plane as shown in FIG. 2.
The parallel inductances L are mostly formed by the conductor posts 7, and their value depends on the length of the conductor posts 7. In the meantime, the series capacitances C are formed between adjoining conductor pieces, and their value depend on the distance between the adjoining conductor pieces and the size of the conductor pieces. It is described in PTL 1 that the HIS provides high impedance to suppress the propagation of surface currents in the vicinity of the resonant frequency of the resonant circuit that is composed of the parallel inductance L and the series capacitance C. PTL 1 also describes that the propagation-suppressed bandwidth (band gap bandwidth) is proportional to the reciprocal of the series capacitance C.
With the product of the series capacitance C and the parallel inductance L unchanged in value as shown in FIG. 2, the conductor pieces 2 can be made smaller to reduce the area occupied by the HIS without changing the center frequency of the band gap.
Since the band gap bandwidth is proportional to the reciprocal of the series capacitance C, the parallel inductance L can be increased to widen the band gap bandwidth while maintaining the product of the series capacitance C and the parallel inductance L unchanged in value.
Several methods have been proposed to increase the parallel inductance L. For example, according to the methods shown in FIG. 13 of PTL 3 and FIG. 17 of PTL 4, the dielectric plate between the conductor pieces 2 and the conductor plane 3 are formed in a two-layer structure as shown in FIG. 3(a) (a first dielectric plate 18 and a second dielectric plate 28). Inductance elements 6 are formed on the lower, second dielectric plate 28. The conductor pieces 2 and the inductance elements 6 are connected by first conductor posts 17. The inductance elements 6 and the conductor plane 3 are electrically connected by second conductor posts 27.
FIG. 3(b) is an equivalent circuit diagram of the structure shown in FIG. 3(a). As is evident from FIG. 3(b), the insertion of the inductance elements 6 increases the parallel inductances L. Examples of the inductance elements 6 include spiral coils 16 shown in FIG. 4(a) and meander coils 26 shown in FIG. 4(b), as well as surface acoustic wave resonators and bulk acoustic wave resonators.
There are known examples of application of the EBG structure. For example, PTL 1 describes that the EBG structure is used as the reflector of an antenna that employs a frequency band within the band gap frequency band of the EBG structure. It is described that such use prevents the propagation of surface waves through the EBG structure, whereby backside emission is suppressed to avoid degradation of antenna characteristics.
It is described that when the EBG structure is used as the reflector of an inverted L antenna in particular, the in-phase reflection of electromagnetic waves on the EBG structure can be utilized to improve radiation efficiency aside from the suppression of surface waves. In addition, the antenna element can be located close to the reflector surface, so that the antenna can be reduced in thickness.
In PTL 3, the EBG structure is used for a grounding cabinet in order to prevent interference between two antennas through surface currents. In PTL 5, the EBG structure is used on a part of a cabinet's inner wall. If various functions are integrated into a high frequency circuit in a cabinet, unwanted electromagnetic radiation occurring in the cabinet can cause electromagnetic interference between the signals of the respective functions and the problem of adverse effects on the characteristics of the entire high frequency circuit.
The use of the EBG structure for the cabinet inner wall that is opposed to the high frequency circuit prevents unwanted electromagnetic radiation in the cabinet. The cabinet inner wall can be brought close to the high frequency circuit with no change in the characteristics of the high frequency circuit, which allows miniaturization of the cabinet.
Moreover, it is known from PTL 6 that the foregoing EBG structure can be used to form a parallel-plate waveguide type EBG structure. A parallel-plate waveguide type EBG device refers to a structure that suppresses the propagation of electromagnetic waves in a parallel-plate waveguide across a certain bandwidth. FIG. 5 shows the parallel-plate waveguide type EBG structure 11 described in PTL 6.
The parallel-plate waveguide type EBG structure 11 includes a first conductor plane 14 and the HIS 1 shown in FIG. 1. The first conductor plane 14 and the HIS 1 are electrically insulated from each other. The periodic array of conductor pieces 2 in the HIS 1 is located in a conductor layer between the first conductor plane 14 and the conductor plane 3 of the HIS 1. The conductor pieces 2 and the conductor plane 3 are electrically connected by respective conductor posts 7.
The parallel-plate waveguide type EBG structure 11 can be considered as a periodic structure of a unit cell 9 which is the component unit.
PTL 6 discloses that if the first conductor plane 14 and the conductor pieces 2 of the HIS 1 are adjacent to each other, the equivalent circuit of the parallel-plate waveguide type EBG structure 11 per unit cell 9 is as shown in FIG. 6, such that the transmission lines are shunted by a series resonant circuit 12 in the center. In FIG. 6, the capacitance C1 represents the capacitance formed between the conductor piece 2 and the first conductor plane 14. The inductance L represents the inductance formed by the conductor post 7 between the conductor piece 2 and the conductor plane 3.
Unlike the HIS 1, the equivalent circuit of FIG. 6 includes no series capacitance C between the thumbtack conductors. The reason is that the adjacency of the first conductor plane 14 and the conductor pieces 2 of the HIS 1 makes the capacitances C1 between the first conductor plane 14 and the conductor pieces 2 dominant over the series capacitances C between the adjoining thumbtack conductors, so that the series capacitances C between the adjoining thumbtack conductors become negligible.
It is described in PTL 6 and others that the parallel-plate waveguide type EBG structure 11 has a band gap frequency band in the vicinity of the resonant frequency of the series resonant circuit 12 shown in FIG. 6. It is also described that the size of the unit cell 9 can be reduced to increase the resonant frequency of the series resonant circuit 12, whereby the stop band is shifted to higher frequencies.
This means that when there is provided the first conductor plane 14 and the distance between the conductor pieces 2 and the first conductor plane 14 is adjusted, it is possible to create a band gap in other than the band gap frequency band of the HIS 1 itself.
Unlike the HIS 1, it is described in PTL 7 and others that the band gap will not be narrowed even if the series capacitance C is increased.
The parallel-plate waveguide type EBG structure 11 can be used, for example, to form the power supply and ground planes of a printed-circuit board (PCB) in electronic equipment, whereby power supply noise associated with the switching operations of active devices mounted on the PCB can be suppressed.
{Citation List}
{Patent Literature}
                {PTL 1} U.S. Pat. No. 6,262,495 B1 (FIG. 1, FIG. 2a, FIG. 2b)        {PTL 2} U.S. Pat. No. 6,483,481 B1 (FIG. 2a, FIG. 3a)        {PTL 3} U.S. Pat. No. 6,933,895 B2 (FIG. 13)        {PTL 4} JP-A-2006-253929 (FIG. 1)        {PTL 5} JP-A-2004-22587 (FIG. 1)        {PTL 6} U.S. 2005/0029632 A 1 (FIGS. 1, 2, 4)        